Portable and stationary reciprocating piston type air compressors are used for many applications, including operating spray guns, air brushes, pneumatic tools, tire inflaters, and the like. In this type of air compressor, a piston is mounted to reciprocate within a cylinder. Typically, the piston is coupled to a motor of the air compressor through an eccentrically rotated crank assembly.
In one type of crank assembly, an eccentric plate is provided with a pair of parallel spaced apart openings formed therethrough. The openings receive ends of a main compressor drive shaft and an eccentric shaft to connect the two shafts together in parallel relationship to one another. Or, the plate can be provided with a single opening formed therein. In this crank assembly, the compressor shaft is usually disposed in the single opening and the eccentric shaft is secured to the plate, such as, for example, by welding. In either type, the compressor drive shaft is in turn coupled to a motor, and the eccentric shaft is coupled through a connecting rod to the piston. In the third design, the eccentric plate and eccentric shaft are cast together. The drive shaft may be secured to the eccentric plate or also may be cast integrally with the eccentric plate. During operation the compressor drive shaft rotates the plate causing the eccentric shaft to be rotated along therewith. As a result, the connecting rod reciprocates the piston within the cylinder.
In the first type of crank assembly, the openings provided in the eccentric plate must be precisely formed to position the two shafts in parallel relationship to one another. This requires that the diameters of the openings and the ends of the two shafts be machined to close tolerances so that no radial movement of the shafts occurs. If radial movement occurs, the two shafts are no longer maintained in an aligned parallel relationship, and undue vibration, noise, and premature component wear will occur during operation of the air compressor. However, it is difficult to consistently maintain tolerances and considerable time and expense are incurred to machine these components.
In the second type of crank assembly, the eccentric shaft must be secured to the eccentric plate so that it is parallel to the opening for the compressor shaft. This requires that the eccentric shaft be carefully positioned on the plate, and, while in this position, that the shaft be secured thereto without movement. Any movement of the eccentric shaft from the desired position on the plate can result in the two shafts not being in parallel alignment with respect to each other. In addition, the point of attachment of the eccentric shaft to the plate must be sufficiently strong so as to prevent failure at this point during operation of the compressor. Furthermore, the diameters of the opening and the compressor shaft must be precisely machined to close tolerances.